A Senate vote to renew an expiring ban on plastic firearms capable of evading metal detectors and X-ray machines is shaping up as a bittersweet moment for gun control supporters, days before the anniversary of the deadly mass shooting at Sandy Hook Elementary School in Connecticut.

Monday’s vote to extend the prohibition on plastic guns for another decade responds to a growing threat from steadily improving 3-D printers that can produce such weapons. But gun control advocates seem sure to lose an effort to impose additional, tougher restrictions on plastic firearms — a harsh reminder of their failure to enact any new federal gun curbs in the year since 20 first-graders and six educators were murdered in Newtown, Conn.